The present invention relates to a sensor to sense dissolved oxygen (DO) in boiler water, and the like, for determining purity of the water. The sensor provides an output current and the present invention teaches reducing background or offset current of the sensor to improve the ability to sense low levels of oxygen in water by controlling the configuration of an internal oxygen diffusion path of residual oxygen in a liquid electrolyte used for the sensor.
The amount of dissolved oxygen in aqueous solutions such as water is a direct indication of water quality for power plant corrosion control applications. The prior art dissolved oxygen sensors are enclosed in a housing that protrudes into a flowing stream or sample of the water and when energized with an electrical potential the oxygen in the water or other liquid will pass through a gas permeable membrane to a component of the sensor, called a cathode.
The gas permeable membrane of prior art sensors cover the sensor components, which are in an outer housing. The cathode which is normally gold, and a silver anode are mounted at spaced locations in the housing. An electrolyte solution, such as potassium chloride, immerses the cathode and anode.
When the sensor does start to operate, that is, when an oxygen reduction potential is applied to the cathode, the oxygen in the electrolyte stored in a chamber in the sensor housing diffuses into a film of electrolyte between the membrane and the cathode. Oxygen in the vicinity of the cathode gets reduced through the following known electrochemical process:O2+2H2O+4e→4OH—  (1)
The process creates an oxygen concentration gradient around the cathode which generates a diffusion of residual dissolved oxygen contained in the liquid electrolyte in the sensor chamber toward the cathode. At steady state a constant residual oxygen flux reaches the cathode and creates the constant background or offset current. The current from constant residual oxygen flux follows Fick's First Law, and the background current Ib can be expressed as:Ib=4FSDAP/L.  (2)
In the above equation (2), F is the Faradaic constant; S the oxygen solubility; D the diffusion coefficient of oxygen through the electrolyte; A the cross sectional area of the diffusion flux, or essentially the cross sectional area of the channel between the cathode and the electrolyte chamber; P the partial pressure of oxygen in air; and L the diffusion channel length. In equation (2) above all of the values of the factors at the right are known. At room temperature F is 95600 C/mole, SD was tested to be 5×10−10 mole/atm.m.s, and P is 0.21 atm.
A, the area of the channel or path provided for diffusion of oxygen flux from the electrolyte chamber to the cathode, and L, the diffusion path length both can be changed through sensor design.
The residual oxygen thus causes an output current, even when the sensor is exposed to oxygen-free media.
When the sensor is used for sensing oxygen in water (or other aqueous solutions) to be monitored the membrane is exposed to the water and dissolved oxygen in the water is also attracted to the cathode and diffuses through the membrane to the film of electrolyte between the membrane and the cathode and then to the cathode.
The output current from dissolved oxygen in water being sampled, above the background or offset current of the sensor, is directly proportional to the oxygen partial pressure in the water, or in other words, proportional to the concentration of dissolved oxygen in the water.
To achieve sensitivity to low levels of dissolved oxygen in the water, in the range of a few parts per billion (ppb), the background or offset current of the sensor must be low. The background current is mostly contributed from the dissolved oxygen that remains in the electrolyte stored in the sensor, which has been termed the residual oxygen. One prior art method to reduce the background current has been to use an extra electrode guard ring to deplete the residual oxygen around the cathode electrochemically during the sensor operation. This adds cost and parts to the sensor.
The residual oxygen cannot be eliminated from the electrolyte. So the problem is to reduce the background current caused by the residual oxygen in the electrolyte with a simple, low cost construction, without sacrificing performance for measuring dissolved oxygen in water samples.